Optical communication is becoming the preferred method for secure, high-bandwidth communications. Fiber-based communication systems, for example, are used in environments where the access points are known and fixed, and free-space communication systems are used in remote applications where access points may vary. For the latter systems, compact, lightweight, field deployable receivers are desired. Yet, the size of existing communications equipment has limited the effectiveness of such devices.
To understand the problems with free-space communications systems, one may look to the environments in which these systems operate. In a standard configuration, a laser transmitter produces an information carrying laser beam that transmits that signal through air. A remote user then uses an optical receiver to detect and demodulate that signal, to obtain an electrical rendition of the original laser signal. Over great travel distances, however, the original laser beam will expand and distort in response to anomalies in the air medium through which the beam travels. Turbulence in the atmosphere, for example, may distort the laser beam and produce a twinkling or blurring effect that represents changes in intensities and phase across the laser beam wavefront.
Additionally, atmospheric turbulence and poor optical quality receivers prevent the laser beam from being focused to a point at the remote location. Rather, the laser beam is only focused down to a blurred spot by the receiver. In other words, although the optical beam may originate from a laser point source, in free-space communication systems, that laser point source is imaged to a two-dimensional blur spot at the remote receiver.
As a result of this blur spot, a larger detector is needed to collect the available energy in the laser signal. In fact, in remote applications, there is so much signal intensity loss over the propagation path that it is desirable to collect as much of the received optical signal as possible, which means that larger diameter optical receivers must be used. Larger optical receivers, however, increase weight and reduce portability—two things undesirable for remote deployable receivers. Larger detectors also slow receiver responsiveness, because the intrinsic capacitance of larger detectors is larger, and scales with the area of the detector which means larger parasitic effects and longer response times. These performance limitations also adversely affect the bandwidth (and thus operating data rates) of optical receivers, preventing them from being used in high data rate applications.